The present invention relates to a process for eliminating halogen-containing compounds, more particularly chlorine-containing compounds, contained in a gas or liquid.
In some industrial applications, halogen-containing compounds, in particular chlorine-containing compounds, contaminate a gas or liquid stream and have to be eliminated.
One illustrative example in the petroleum industry is the elimination of halogen-containing compounds, in particular chlorine-containing compounds, contained in the gas or liquid originating from catalytic reforming.
One aim of catalytic reforming is to produce hydrocarbons with an increased octane number. It has been established that the octane number of a hydrocarbon is higher if it is branched, cyclic or aromatic. Thus hydrocarbon cyclisation and aromatisation reactions are encouraged.
Normally, such hydrocarbon cyclisation and aromatisation reactions take place in the presence of heterogeneous chlorine-containing bimetallic catalysts. Such chlorine-containing catalysts are based on alumina and usually comprise platinum and another metal such as tin, rhenium or iridium. The presence of chlorine in said catalysts is important since, added to the alumina, it provides the system with overall acidity and contributes to re-dispersing the platinum over time, thus stabilising the catalytic activity of the catalyst.
However, adding chlorine is not a solution without disadvantages. Over time, chlorine clutes, in particular in the form of HCl. Such elution results firstly in the constant necessity to recharge the catalyst with chlorine. It also leads to the presence of HCl and other chlorine-containing compounds in gaseous and liquid effluents from catalytic reforming, which can lead to a corrosion problem in the facility and to the formation of other unwanted products which are deleterious to the operation of downstream units.
Catalytic reforming also produces hydrogen. When refining petroleum, hydrogen is a particularly precious product, in particular for use in hydrotreatments which are becoming ever more developed with the aim of environmental protection.
At the outlet from a conventional catalytic reforming process, which operates under a pressure of about 20 bars or more, the gaseous effluents are mainly composed of hydrogen, light hydrocarbons such as methane, ethane, etc., and generally contain traces of HCl and water. It is thus important to be able to eliminate all traces of HCl from such effluents, and then to recycle and use the purified hydrogen, still in the refinery.
Further, regenerative processes or new generation, have recently been developed and are expanding in that field. Such processes operate at a pressure of about 3 to 15 bars or less.
At the outlet from the regenerative catalytic reforming step, light hydrocarbons, traces of HCl and water, traces of unsaturated hydrocarbons such as ethylene, propylene, butene, butadiene etc. have been detected in addition to hydrogen. In the presence of chlorine and in contact with alumina, such unsaturated hydrocarbons are at least partially transformed into organochlorinated compounds which in turn, after many reactions with other organochlorinated compounds and/or unsaturated compounds, lead to the formation of high molecular weight oligomers known as green oils. Such green oils can cause blockages in the facility. Further, a significant drop in the service life of the adsorbent has been observed: in some cases, a drop of 4 to 5 times has been observed.
In that type of process, it is important to be able to eliminate all traces of HCl from such effluents in order to be able to recycle and thus use the purified hydrogen, and to reduce or prevent the formation of green oils.
The aim of the present invention is to provide an improved process for efficiently eliminating halogen-containing compounds in general, chlorine-containing compounds in particular, and HCl more particularly, contained in a gas or liquid.
A further aim of the present invention is to provide a process which uses a composition which will substantially reduce or prevent the formation of halogen-containing oligomers, in particular chlorine-containing oligomers known as green oils, downstream of regenerative reforming processes or new generation.
The invention achieves these aims in providing a process for eliminating halogen-containing compounds contained in a gas or a liquid.
Throughout the following text, the term xe2x80x9cprocess for eliminating halogen-containing compoundsxe2x80x9d means xe2x80x9cprocess for eliminating, reducing and/or preventing the formation of halogen-containing organic or inorganic compounds including higher weight oligomersxe2x80x9d.
Thus the present invention provides a process for eliminating halogen-containing compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition based on an alumina and/or a hydrated alumina and at least one compound (A) comprising at least one metallic element selected from metals from groups VIII, IB and/or IIB of the periodic table, and in that the total metallic element(s) content is at most 45% by weight with respect to the total composition weight. More particularly, the complement by weight of the composition comprises in a major part alumina and/or a hydrated alumina.
Said composition is an adsorbent on which halogen-containing compounds, i.e. halogen-containing organic compounds and halogen-containing inorganic compounds, are retained by absorption. These halogen-containing compounds are eliminated from said gas or liquid under solely adsorption conditions, leading then to the entire purification of the gas or the liquid as no halogen-containing compounds (halogen-containing organic and inorganic compounds) are detected downstream the process. The process of the invention is implemented under reducing conditions, in the presence of hydrogen and/or hydrocarbon in the medium. By reducing conditions, we mean a medium substantially devoid of oxygen or any other oxidizing agent (i.e. less than 0.1% vol of O2 or any other oxidizing agent). In case the process of the invention is carried out in the presence of a small amount of oxygen, oxygen will have no oxidizing power and no effect on the implementation of the process since halogens are adsorbed on the adsorbent composition. Oxygen, if there is some, comes from the contaminated gas or liquid. No oxygen is introduced into the process of the invention by external means.
Throughout the present text, the periodic table used is that from the xe2x80x9cSupplement au Bulletin de la Socixc3xa9txc3xa9 Chimique de France, No. 1, January 1966xe2x80x9d.
The composition for the adsorption of halogen-containing compounds and used in the process of the present invention can be in the form of a powder, beads, extrudates, crushed material or monoliths.
The first essential constituent of the absorbent composition is alumina, a hydrated alumina or a mixture of an alumina and a hydrated alumina.
The starting alumina generally has a specific surface area of at least 5 m2/g, preferably at least 10 m2/g and more preferably at least 30 m2/g.
In the present invention, all of the specific surface areas indicated are surface areas measured using the BET method. This means the specific surface area determined by nitrogen adsorption in accordance with American standard ASTM D 3663-78 established from the BRUNAUER-EMMETT-TELLER method described in xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d 60, 309 (1938).
The starting alumina can also have a total pore volume (TPV) of at least 0.10 cm3/g, preferably at least 0.20 cm3/g, more preferably at least 0.25 cm3/g. This total pore volume is measured as follows: the values of the grain density and absolute density are determined: the grain (Dg) and absolute (Da) densities are measured using a picnometry method using mercury and helium respectively. The TPV is given by the formula:
[l/Dg]xe2x88x92[l/Da]
The processes for preparing the aluminas with the total pore volume and specific surface area characteristics necessary for carrying out the process of the invention are known to the skilled person.
Regarding the alumina, the alumina powder used as a starting material to prepare the composition of the invention can be obtained using conventional processes such as gel precipitation, and rapid dehydration of an alumina hydroxide (or hydrated alumina) such as Bayer hydrate (hydrargillite). This latter alumina is preferred.
Alumina beads can be formed by drop coagulation. This type of bead can, for example, be prepared as described in European patents EP-A-0 015 801 or EP-A-0 097 539. The porosity can be monitored using the process described in EP-A-0 097 539 by drop coagulation of a suspension or an aqueous dispersion of alumina or a solution of a basic aluminium salt in the form of an emulsion constituted by an organic phase, an aqueous phase and a surfactant or an emulsifying agent. Said organic phase can in particular be a hydrocarbon, the surfactant or emulsifying agent is, for example, Galoryl EM 10(copyright).
Alumina can also be obtained in the form of beads by agglomeration of an alumina powder. Agglomeration into the form of beads is carried out directly on the alumina powder by rotary technology. The term xe2x80x9crotary technologyxe2x80x9d means any apparatus in which agglomeration is carried out by contact and rotation of the product to be granulated on itself. Apparatus of this type includes the rotary bowl granulator, and the rotary drum. This type of process can produce beads with controlled sizes and pore distributions, these dimensions and distributions generally being created during the agglomeration stage. The volumes of pores with a given diameter can also be controlled during this agglomeration step by suitable regulation of the rate of introduction of the alumina powder and possibly of water, the apparatus rotation rate or by introducing a seed.
Alumina extrudates can be obtained by mixing then extruding an alumina based material, said material possibly originating from rapid dehydration of hydrargillite or precipitation of an alumina gel. The porosity of the extrudates can be controlled by the mixing operation conditions for the alumina before extrusion. The alumina can also be mixed with pore-forming agents during mixing. Examples are the extrudates prepared using the process described in United States patent U.S. Pat. No. 3,856,708.
Crushed alumina material can originate from crushing any type of alumina based material such as the beads obtained by any type of process (drop coagulation, rotary bowl granulator or rotary drum) or from extrudates. The porosity of the crushed material can be controlled by selecting the alumina based material to be crushed to obtain them.
Regardless of the form of the alumina, the porosity can be produced by different means such as the choice of granulometry of the alumina powder or the mixture of a plurality of powders with different granulometrics. A further method consists of mixing a compound, a pore-forming agent, with the alumina powder, before or during the agglomeration or extrusion stages, the compound completely disappearing by heating and thus creating the porosity in the alumina.
Examples of pore-forming agents which can be used are wood flour, wood charcoal, sulphur, tars, plastics materials or emulsions of plastics materials such as polyvinyl chloride, polyvinyl alcohols, naphthaline or the like. The quantity of pore-forming agents added is not critical and is determined by the desired pore volume.
Following forming, the alumina obtained can undergo different operations intended to improve its mechanical strength, such as ageing by keeping it in an atmosphere with a controlled humidity followed by calcining then impregnating the alumina with a solution of one or more acids and a hydrothermal treatment in a confined atmosphere.
Finally, after these different treatments, the alumina can be dried, then optionally calcined.
As indicated above, the alumina used as the starting material for preparing the composition of the invention can be obtained by rapid dehydration of a hydrated alumina such as Bayer hydrate (hydrargillite).
This hydrated alumina can also be used directly as a starting material to prepare the composition of the invention. Advantageously, the hydrated alumina is hydrargillite.
When the starting material is hydrated alumina, a binder can be added to the composition to ensure satisfactory mechanical properties. Binders can, for example be based on clay, such as attapulgite, kaolinite, or bentonite.
In the present invention, the hydrated alumina can have a specific surface area or 5 m2/g, preferably over 10 2/g. It can also have a total pore volume (TPV) of at least 0.10 cm3/g.
The hydrated alumina can also be characterised by its loss on ignition (LOI), measured at 300xc2x0 C., which is advantageously more than 5%, or even more than 10%.
The loss on ignition (LOI) is determined in accordance with the AFNOR standard NF T20-203, October 1973 xe2x80x94EQV ISO 803.
The different forming processes described above for alumina can also be used for hydrated alumina.
A mixture of alumina and hydrated alumina can also be used.
The second constituent in the composition is the doping element, more precisely the metallic element, provided by compound (A).
The composition used in the process of the invention can comprise one or more metallic elements selected from metals from groups VIII, IB, and/or IIB of the periodic table.
Introduction of a metallic element, on or into the alumina and/or hydrated alumina can be carried out using any method which is known to the skilled person. This introduction is preferably effected by depositing the metallic element(s) onto the alumina and/or hydrated alumina.
The metallic element can, for example, be introduced by impregnating the prepared alumina and/or hydrated alumina with at least one compound (A) comprising at least one metallic element or by mixing at least one compound (A) comprising at least one metallic element with alumina and/or hydrated alumina downstream or when forming of the latter.
The doping element can also be introduced into the alumina and/or hydrated alumina by co-precipitating the alumina and/or hydrated alumina and at least one compound (A) comprising at least one metallic compound.
When introducing by impregnation, this can be carried out in known manner by bringing the alumina and/or hydrated alumina into contact with a solution, a sol or a gel comprising at least one doping element in the form of the oxide or salt or a precursor thereof.
The operation is generally carried out by immersing the alumina and/or hydrated alumina in a set volume of a solution of at least one precursor of at least one doping element. The term xe2x80x9csolution of a precursor of a doping elementxe2x80x9d means a solution of at least one salt or at least one compound of the doping element or elements, the salts and compounds being thermally decomposable.
The concentration of salt in the solution is selected as a function of the doping element to be introduced into the alumina and/or alumina hydrate and on the final amount of doping element required.
The doping element impregnation surface is determined by the compound of solution adsorbed. Thus the volume of doping element adsorbed is equal to the total pore volume of the alumina and/or hydrated alumina to be impregnated. It is also possible to impregnate the alumina and/or hydrated alumina by immersing it in a solution of the precursor of the doping element and eliminating the excess solution by draining.
In a preferred implementation, the doping element is introduced by dry impregnation, i.e., impregnation is carried out with just the volume of solution necessary for said impregnation, with no excess.
Compounds (A) acting to introduce at least one metallic element selected from metals from groups VIII, IB and/or IIB of the periodic table into the alumina and/or hydrated alumina can be selected from organic or inorganic compounds. They are preferably selected from inorganic compounds.
More particularly, the term xe2x80x9cinorganic compoundsxe2x80x9d means inorganic salts such as carbonates, bicarbonates, cyanides, cyanates, alkoxylates, hydroxides, sulphates and nitrates.
As mentioned above, said compounds comprise at least one metallic element selected from the following metals:
group VIII: iron and nickel;
group IB: copper; and
group IIB: zinc.
Compounds (A) are preferably selected from nitrates, sulphates, hydroxides, carbonates and bicarbonates of iron, nickel, copper or zinc, used alone or as a mixture.
The composition used in the process of the invention is obtained by heat treating the alumina and/or hydrated alumina after introducing compound or compounds (A). Heat treatment is carried out at a temperature determined as a function of the nature of the doping element or elements.
An alumina and/or hydrated alumina is used which, after introducing at least one compound (A) comprising at least one metallic element as cited above, can be heat treated at a temperature of at least 100xc2x0 C. This heat treatment can preferably be carried out at a temperature in the range 150xc2x0 C. to 600xc2x0 C., more preferably in the range 200xc2x0 C. to 550xc2x0 C.
The duration of heat treatment is not critical in itself. It will depend on the temperature: generally, the higher the temperature, the shorter the treatment period.
When introducing the compound or compounds (A), the concentration of the solution of the compound is selected such that the total content of the metallic element or elements is at most 35% by weight, more particularly at most 25% by weight, with respect to the total composition weight.
This content is at least 0.005% (50 ppm) by weight, preferably at least 0.5% by weight with respect to the total composition weight, the metallic element not being iron.
When at least one of the metallic elements is iron, the total iron content is at least 0.1% by weight, preferably at least 0.5% by weight, more preferably at least 0.7% by weight, with respect to the total composition weight.
More particularly, the total content of the metallic element or elements is in the range 0.5% to 20% by weight, more particularly still in the range 0.7% to 15% by weight, with respect to the total composition weight.
The content of alumina and/or hydrated alumina is more particularly at least 35% by weight with respect to the total composition weight.
A variation of the present invention consists in a process for eliminating halogen-containing compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition based on an alumina and/or hydrated alumina as described above which also comprises at least one compound (B) comprising at least one element selected from alkalis, alkaline-earth elements, and rare earth elements.
In this variation, the composition used comprises both one or more metallic doping elements selected from metals form groups VIII, IB and/or IIB of the periodic table, and one or more alkali, alkaline-earth or rare earth doping elements.
Compounds (B) acting to introduce into the alumina and/or hydrated alumina, at least one element selected from alkali elements, alkaline-earth elements and rare earth elements, can be selected from organic or inorganic compounds. Preferably, inorganic compounds are used.
More particularly, the term xe2x80x9cinorganic compoundsxe2x80x9d means inorganic salts such as carbonates, bicarbonates, cyanides, cyanates, alkoxylates, hydroxides and nitrates.
As mentioned above, compounds (B) comprise at least one element selected from alkali metals, in particular lithium sodium, potassium, rubidium and caesium; alkaline-earth metals in particular magnesium, calcium, strontium and barium; and rare earth elements, in particular cerium, praseodymium and lanthanum.
In a particular implementation of the invention, compounds (B) are preferably selected from sodium and potassium nitrates, hydroxides, carbonates, and bicarbonates.
The alkali, alkaline-earth and rare earth elements can be introduced into the alumina and/or hydrated alumina using any method known to the skilled person, in particular as described above.
Compounds (A) and (B) can be added before, during and/or after forming the alumina and/or hydrated alumina.
However, compounds (A) and (B) can be introduced using three distinct methods.
The first method, which is the preferred method, consists of using a composition obtained by introducing:
i) firstly, compound or compounds (B) into the alumina and/or hydrated alumina, preferably by impregnation, followed by heat treatment carried out at a temperature of 100xc2x0 C. or more;
ii) then compound or compounds (A) into the alumina and/or hydrated alumina obtained from i) preferably by impregnation, followed by a fresh heat treatment at a temperature of 100xc2x0 C. or more.
More particularly, in this first method a composition is used which is obtained by introducing:
i) firstly, compound or compounds (B) into the alumina and/or hydrated alumina, preferably by impregnation followed by heat treatment carried out at a temperature in the range 200xc2x0 C. to 1200xc2x0 C., preferably 300xc2x0 C. to 1000xc2x0 C.;
ii) then compounds (A) into the alumina and/or hydrated alumina obtained from i) preferably by impregnation, followed by a fresh heat treatment carried out at a temperature in the range 150xc2x0 C. to 600xc2x0 C., preferably in the range 200xc2x0 C. to 550xc2x0 C.
The second method consists of using a composition obtained by introducing:
i) firstly, compound or compounds (A) into the alumina and/or hydrated alumina, preferably by impregnation, followed by heat treatment carried out at a temperature of 100xc2x0 C. or more;
ii) then compound or compounds (B) into the alumina and/or hydrated alumina obtained in i) preferably by impregnation, followed by a fresh heat treatment carried out at a temperature of 100xc2x0 C. or more.
More particularly, in this second method a composition is used which is obtained by introducing:
i) firstly, compound or compounds (A) into the alumina and/or hydrated alumina, preferably by impregnation followed by heat treatment carried out at a temperature in the range 150xc2x0 C. to 600xc2x0 C., preferably 200xc2x0 C. to 550xc2x0 C.;
ii) then compounds (B) into the alumina and/or hydrated alumina obtained from i) preferably by impregnation, followed by a fresh heat treatment carried out at a temperature in the range 200xc2x0 C. to 1200xc2x0 C., preferably in the range 250xc2x0 C. to 1000xc2x0 C.
The third method consists of using a composition obtained by simultaneously introducing compounds (A) and (B) into the alumina and/or hydrated alumina, preferably by impregnation, followed by heat treatment carried out at a temperature of 100xc2x0 C. or more.
More particularly, in this third method, a composition is used which is obtained by simultaneously introducing compounds (A) and (B) into the alumina and/or hydrated alumina, preferably by impregnation, followed by heat treatment carried out at a temperature in the range 150xc2x0 C. to 1200xc2x0 C., preferably 200xc2x0 C. to 1000xc2x0 C.
It is possible to repeat the introduction operations with the same alumina and/or hydrated alumina, and successively introduce a plurality of compounds (A), and if necessary a plurality of compounds (B), into the same alumina and/or hydrated alumina.
Regardless of the selected introduction method, the total content of the alkali, alkaline-earth and rare earth elements is in the range 0.01% to 50% by weight, preferably in the range 0.1% to 40% by weight, with respect to the total composition weight.
The total content of the alkali, alkaline-earth and rare earth elements is advantageously in the range 1% to 40% by weight, more particularly in the range 1.5% to 25% by weight, with respect to the total composition weight.
The specific surface area of the final composition, independently of the nature of the doping element(s), is at least 1 m2/g, preferably at least 5 m2/g, and more preferably over 15 m2/g.
More particularly, the final composition is characterized in that it comprises at most 35% by weight of metallic element selected from metals from groups VIII, IB and/or IIB and between 1.5% and 25% by weight of alkali, alkaline-earth and rare-earth elements, with respect to the total composition by weight.
In another embodiment, the final composition is characterized in that it comprises at most 25% by weight of metallic element selected from metals from groups VIII, IB and/or IIB and between 1% and 40% by weight of alkali, alkaline-earth and rare-earth elements, with respect to the total composition by weight.
The process of the invention is more particularly intended for eliminating chlorine-containing compounds in general, and more particularly for eliminating HCl present in a gas or a liquid.
When the process of the invention is used downstream of a regenerative catalytic reforming or new generation process, HCl elimination is accompanied by a substantial reduction in and/or prevention of the formation of chlorine-containing oligomers or green oils which are also present in the stream.
The process of the invention is advantageously implemented at temperatures not higher than 300xc2x0 C., more preferably not higher than 200xc2x0 C. and even more preferably not higher than 100xc2x0 C. The most preferred temperature range is 0-60xc2x0 C. As described above, at the outlet from the catalytic reforming step, the gaseous effluents are mainly composed of hydrogen, saturated hydrocarbons, traces of unsaturated hydrocarbons (in regenerative catalytic reforming), traces of halogen-containing compounds, and water. When the effluents contain water, the water content is generally in the range 1 to 50 ppm by volume, at the pressure of the unit. Under these conditions, the HCl content, for example, is often in the range 0.2 to 30 ppm by volume.
The process of the invention is also suitable for eliminating halogen-containing compounds contained in a gas or liquid which is free of water, and also in a gas or liquid containing water.
The term xe2x80x9cfree of waterxe2x80x9d means a water content of less than 1 ppm at the pressure of the unit.
The following examples illustrate the invention without limiting its scope.